In the document and product security industry, it is desirable to have a low-cost anti-counterfeiting device that can be used easily by any consumer. Ideally, the device is inexpensive to make but impossible to duplicate, and easy to verify but applicable to large numbers of different documents and products. Known anti-counterfeit measures do not sufficiently satisfy these conditions. Therefore a need exists for low-cost, easy to use anticounterfeit devices that provides a high level of security.
A grating layer on a surface shows interesting properties. The effective refractive index of a grating layer on a surface depends on the grating orientation and grating profile, as well as on the bulk properties of the material composing the layer. Specifically, when the grating period is significantly smaller than the wavelength of light incident on the grating or shorter than half a wavelength of a selected color of light, the effective refractive index is higher if the electric field vector of the light is aligned with the grating lines rather than being perpendicular to the grating lines. If in addition the grating layer comprises a metallic material, the complex reflectivity of the layer depends on the alignment of the electric field vector relative to the grating lines. “Complex reflectivity” refers to the phase shift as well as the amplitude change in each polarization component of the reflected light. In the first case, the grating layer is birefringent, and in the second case the grating layer is a reflective polarization modifier. Both are referred to herein as cases of “form birefringence”, which herein means both the property of altering the polarization of light by differently retarding light of different polarizations, and the property of altering the polarization of light by differently absorbing or reflecting light of different polarizations, as a result of the profile shape of a surface on a subwavelength scale.
Form birefringence has been demonstrated and reported by several researchers, and is exploited commercially. For example, Nano-Opto Corporation sells a “Subwave Polarization Beam Splitter/Combiner” and a “Broadband Polarizer For Optical Networking Applications” that employ surface relief gratings having a period smaller than an optical wavelength.
When illuminating a form birefringent grating structure with polarized light and viewing the reflected (or transmitted) light, color effects can be observed that depend on the orientation of the grating structure with respect to the polarizer directions and the depth of the grating structure. Due to the combination of color and polarization properties of the form birefringent grating structure, it is difficult to duplicate or reverse engineer it. Furthermore, verification is easy by employing standard linear or circular polarizers.